An n-p-.pi.-p.sup.+ avalanche photodiode (APD) comprises a body of .pi.-type conductivity silicon (Si) having an n-type conductivity region extending a distance into the body from a portion of a first surface thereof with a p-type conductivity region extending a further distance into the body from the n-type region, and a p-n junction therebetween. A p.sup.+ -type conductivity region extends a distance into the body from a surface opposed to the first surface. Electrical contact is made to the n- and p.sup.+ -type regions.
In the operation of this APD, a reverse bias is applied to the electrical contacts producing an electric field within the APD whose profile depends upon the impurity concentration in the different regions and which forms a depletion region typically reaching through the .pi.-type region. Light incident on the surface containing the p.sup.+ -type region enters the photodiode and is absorbed primarily in the .pi.- or p-type regions, generating electron-hole pairs. The electrons are accelerated by the electric field until they attain sufficient energy for multiplication which typically occurs within one to three micrometers (.mu.m) of the p-n junction. Holes generated within the high field region are accelerated in the opposite direction and can also undergo multiplication where the electric field is sufficiently high.
One of the limitations of such an APD is that the multiplication process is noisy due to the width of the probability distribution of gains that a carrier can undergo. Webb et al., in RCA Review 35, 234 (1974) disclose that, to a good approximation, the excess noise factor F can be expressed by EQU F=k.sub.eff &lt;M&gt;+(1-k.sub.eff)(2-1/&lt;M&gt;)
where k.sub.eff is a weighted average of the ratio of the hole ionization coefficient to the electron ionization coefficient and &lt;M&gt; is the average avalanche gain. For a Si APD a typical value for k.sub.eff is between about 0.015 and about 0.1 and depends strongly upon both the electric field and its profile. To minimize the excess noise, k.sub.eff must be as low as possible; i.e., the hole multiplication must be minimized and the electron multiplication must be maximized. Thus the electric field should be large where the electron current is highest and hole current is lowest while the field should be small where the hole current is highest.
It is known that, for a given set of n-type and p-type doses but with varying diffusion parameters, the product k.sub.eff .times.V.sub.a .apprxeq.2.35 for such APDs where k.sub.eff &lt;&lt;1 and V.sub.a is the voltage drop across the multiplication region when that region is just depleted. If this relationship is valid for higher values of V.sub.a, it should be possible to achieve a low k.sub.eff merely by choosing diffusion times which would give considerably higher values of V.sub.a. It would be desirable, however, and it is an object of this invention, to reduce the value of k.sub.eff without increasing the voltage drop V.sub.a as greatly as would appear to be necessary from the disclosure of Conradi et al.